Technical Yearbook 2023

approximately 4 mm thick. The residence time of wine in a depth filter is approximately 12 seconds depending on the wine flow and pressure. Contrary to that, a membrane is much thinner, with a much lower residence time. Depth filtration and cross-flow filtration differ considerably regarding their impact on, especially, the colloidal contents of the wine. Depth filtration of 1 - 2 µm is gentle and slow, requiring only 0.5 bar (50 kPa) to discharge filtrate compared with cross-flow filtration of 0.2 µm where more than 2 bar (200 kPa) pressure is needed to produce filtrate. This can affect the filterability of especially red wines with high colloidal content. The retained wine in cross-flow filtration also gets gradually dirtier as the process progresses and more pressure needs to be applied to continue filtration. The same situation occurs with depth filtration but not to the same extent, and due to the slower and more gentle process, the influence on the colloidal structure of the wine is limited. Filter grade options and particulate distribution Depth filtration offers different filtration grades. A sheet with a nominal porosity of 2 - 3 µm will remove all particles between 2 - 5 µm. If particles smaller than 2 µm need to be removed, another grade sheet should be used. A wine containing a particulate distribution ranging from small (1 - 5 µm) to big (5 - 12 µm) will block the filter quickly by surface covering if a too low porosity grade sheet is used, while a higher porosity grade will let the smaller particles through. In such a case, two successive filtrations of different filtration grades need to be used, or a cross-over plate in the same filter can be used. An alternative filtration option for all of these examples is cross-flow filtration, which will remove all particles bigger than 0.2 µm because that is usually the nominal pore size of cross-flow membranes. However, some winemakers believe that removing all particles from red wine negatively impacts the wine structure and instead use a medium or coarse Unfortunately, the particular distribution of wines is not known before filtration. Two laboratory methods, NTU (nephelometer turbidity units) and FI (filterability index) can be used to indicate the degree of turbidity in wines. NTU is an approximate of the particular load of a wine and is based on the light reflection of the particles in wine. The more particles, the higher the NTU will be. However, it does filtration before bottling their red wines. Measurement of particular distribution

not indicate the type of particles nor measure the presence of dissolved material or colloids like glucans, mannoproteins, carboxymethylcellulose (CMC), gum Arabic and tannin protein associations. Generally, wine is suitable for sterile filtration if its NTU is < 1. The FI of a wine is determined by measuring the time difference (in seconds) in the filtration of 200 and 400 mL wine, respectively, through a membrane with a selected pore size by applying 200 kPa pressure. The lower the time difference between the two wine volumes, the better the filterability of the wine. The assumption that a significant correlation between the NTU and FI exists is not valid. The influence of colloids on cross-flow filtration Wine with high colloidal content, like red wines in comparison with white wines, has a higher potential for a variation in filterability during cross-flow filtration. As mentioned, depth filtration is gentler than cross-flow filtration due to the higher porosity and lower pressure applied. As a result, depth filtration exhibits better filterability over time. It appears that wine components and additives of colloidal nature, like colouring matter, tannins, proteins, gum Arabic, mannoproteins and grape juice concentrate just before cross-flow filtration, exacerbate the potential of colloidal instability. Monitoring cross-flow performance Any negative impacts on filtration before packaging cause downtime in the process and must be prevented. Due to the difference between NTU and FI measurements, both need to be applied to ensure proper filtration. For example, a wine can have an acceptable NTU reading of < 1, yet exhibit poor filterability. A FI is consequently essential in such a case. As a general rule, a proper cross-flow filter should deliver a filtrate with a FI of fewer than 5 seconds. When filtration is blocked in a sequence of filters, the preceding filter’s performance must be checked first. Conclusion Both depth filters and cross-flow filters can play a role in wine filtration. Different factors like the wine condition, desired outcome, cost and equipment availability influence the choice. Cross-flow filtration is also not a guarantee of no filtration problems and must be regularly checked for its performance.  Reference https://www.wineland.co.za/the-impact-of-cross-flow filtration-on-wine-filterability/

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TECHNICAL YEARBOOK 2023